Time-Dependent Rheology of a Model Waxy Crude Oil with Relevance to Gelled Pipeline Restart

Abstract

When ambient temperatures are low, paraffinic crude oils being transported in pipelines may form gels composed of wax crystals. If pipeline flow ceases, these waxy gels may make it difficult to restart the flow without breaking the pipe. To predict the severity of this problem, we consider the rheology of a transparent model waxy crude oil for which pipeline flow visualization results are presented elsewhere. We investigate characteristics of the model oil determined by cone-plate shear flow measurements, such as the viscosity and wax appearance temperature, the gelation temperature, the elastic modulus, and the yielding behavior of the gel. The yielding behavior is a critical determinant of pipeline restart, and the time-dependent yielding behavior observed for this model oil is similar to that reported previously for North Sea crude oils. In particular, at sufficiently low-stress levels, the gel never yields, whereas the gel yields or “fractures” immediately at sufficiently high-stress levels. At intermediate-stress levels, the gel “creeps” with a delay time to fracture that ranges from seconds to hours, depending upon the imposed stress value. Some authors have suggested that waxy gels slowly degrade as they creep and that this gives rise to the very long delay times to fracture that may be observed. However, a creep-response hysteresis test on the model oil studied here shows that the gel elastic modulus does not vary with time during creep, a result which is inconsistent with the degradation mechanism.